His death took place in mysterious circumstances, at Arundel House in London. It was announced to have been due to an 'ague' which developed into gangrene. However, Lucy Hutchinson was recorded as having heard that Onslow, whom she regarded as her recently imprisoned husband's enemy for denouncing the latter in parliament for his role as a Regicide, had been struck by lightning, an allegation widely believed in Onslow's family.[1]

Sir Richard married Elizabeth Strangeways (c.1601 – 27 August 1679), daughter and heir of Arthur Strangeways. They had fourteen children: